Advances in analytical techniques make it feasible to accurately determine the concentration of most metal ions and salts in a plating solution. However, the analysis of organic plating additives (typically in the ppm level), contaminants from drag-in, and plating reaction by-products, is usually very difficult, if not impossible. Even when "complete" analysis is possible for a plating solution, there is always a concern about "unknown species" that may affect the plated deposit. In practice, in addition to the analytical technique, other methods are used for routine checking of the overall performance of plating solutions.
Hull cell has been recognized as one of the most important tools to monitor overall performance of plating solutions. Hull cell was first described by R. O. Hull in a paper entitled "Current Density Range Characteristics, Their Determination and Application", Proc. Amer. Electroplates' Soc., 27 (1939) pp. 52-60. Also see U.S. Pat. No. 2,149,344 issued on Mar. 7, 1939 to R. O. Hull. One of the principle advantages of Hull cell measurements is that it is possible to assess the deposit characteristics at various current densities on a single test panel. It is also possible to carry out evaluations using various temperatures, solution compositions, addition agents, contaminants, etc.
Essentially, a Hull cell is an electro-plating cell with a particular trapezoidal geometry (FIG. 6). A flat cathode, 61, is fixed at an angle to a flat anode, 62, within a box-like container, 63, holding an electrolyte, 64. Both the cathode and the anode occupy the full cross-section of the cell. Several sizes of Hull cells are commercially available with solution capacities of 250, 267, 320, 534 and 1000 cc. The principle behind the Hull cell is to create a variation of solution resistance between the electrodes and to use a current restricting angle between the cathode and an insulating plane. The above arrangement produces a large variation of current density across the deposition on the test panel. Agitation within the Hull cell is usually provided by an external paddle, magnetic stirring bar or by forcing air through the electrolyte in the vicinity of the cathode. However, the results are usually less meaningful due to poorly reproducible mass transfer characteristic between experimental runs. Electrochemical reaction kinetics are usually greatly influenced by trace amounts of organic/inorganic additives and the concentration of metal ions and salts. Therefore, different degrees of agitation near the plating object will result in large variations in deposit properties. Unfortunately, the traditional Hull cell does not provide a reproducible mass transfer and, therefore, can only be used for qualitative process control.
The intention of a Hull cell is to create large variations of current density over the test panel, typically over one order of magnitude. It is conceivable that at the high current density region, the plating reaction is mass transfer limited or nearly mass transfer limited. A deposit obtained from a mass transfer limited condition has a different structure when compared to a deposit plated at conditions that are not mass transfer limited. Therefore, it is difficult to determine whether an irregular deposit is the result of a change of plating variables or is simply caused by irreproducible mass transfer.
Thus, there is a need for a new design of a cell which permits assessment of deposit characteristics at various current densities on a single test panel and which also has reproducible mass transfer and could be useful for quantitative measurements.